What Nursey says is true. There is no limit, but you'll have to prove that you made the cruise, and this can be done in almost any way. I, for instance, got them to count several really old cruises, when I came up with the old "sign-n-sail" cards, that I'd stuck on a bulletin board, and forgot about. I've heard of others who faxed them the copy of their "boarding photo" in front of the "Mardi-Gras" sign. Bottom line is, if you have any proof at all that you took the cruise, they'll count it.
Good luck, and welcome aboard.